Lasers have been used to provide heat, directly or indirectly, for the purpose of removal of plaque or other obstructing materials in a circulatory lumen. One such system is disclosed in Loeb U.S. Pat. No. 4,445,892 (the '892 Patent) entitled "Dual Balloon Catheter Device". Another is disclosed in Hussein et al. U.S. Pat. No. 4,773,413 entitled "Localized Heat Applying Medical Device".
In the system of the Loeb '892 Patent, radiant energy output from a laser is directed by a prism at a 90.degree. to vaporize plaque obstructing a blood vessel. The radiant energy output from the distal end of a fiber optic member is used to directly heat and destroy the obstructing material.
In another embodiment disclosed in the Loeb '892 Patent, the relatively small diameter of the optical fiber produces a similar sized output beam which defines the diameter of the channel made thereby. Lens systems to expand the laser ablation area are not practical for the power density rapidly declines as the area of laser exposure increases.
The noted Hussein et al. Patent discloses the use of an enlarged metallic head mounted at the distal end of the optical fiber. The head is heatable by a beam of laser light. The heated, enlarged head is pressed against the obstructing material to heat and destroy same.
The enlarged head provides a vehicle for creating a flow channel having a diameter greater than the diameter of the heating beam. A heatable head has the disadvantage that the entire head must be heated but only a portion of it is effective as a remover of obstructing material. Hence, a portion of the provided radiant energy does not contribute to making the channel. In addition, the excess heat may damage, in the small arteries, the vessel wall underlying the plaque, or may cause the inner lumen of the vessel to be excessively thrombogenic; although the mechanisms of such phenomena are not yet understood.
While the systems disclosed in the above two patents are useful for the removal of plaque or other obstructing materials, it would be desirable to be able to increase the diameter of the channel being formed by the use of directly applied radiant energy without the need for a lens system and without the need for energy absorbing metal heads.
Laser energy at specific wavelengths, delivered in rapid pulses or continuously, has been shown to destroy tissue and plaque without thermal damage to the vessel wall underlying the plaque. Again, the channel produced is dependent upon the size of the optical fiber.
A number of very small diameter fibers, extended in parallel with one another, can be used to create a usable channel by producing overlapping incident laser energy beams. However, this increases the risk of fiber breakage and incomplete and/or uneven ablation of the obstruction.
In addition, it would be desirable to be able to use rapidly pulsed lasers as energy sources to vaporize obstructions. Pulsed lasers are advantageous in that tissue damage and trauma due to heating can be minimized.
Thus, there continues to be a need to provide a cost effective catheter for direct delivery of radiant energy, such as laser light, onto the surface of plaque or other obstructing material so as to create a channel with a diameter that is larger than the diameter of the beam without diminishing the power density of the beam at the incident area of the obstructing material. In addition, it would be desirable to be able to precisely apply this output radiant energy beam to a selected region within the lumen.
Preferably, the output beam will be directed at least partially ahead and outside of the plane or axis of the distal end of the catheter, so that a channel which is larger than the diameter of the catheter can be formed in a fully occluded lumen. Finally, the catheter should have as small a diameter as possible to facilitate insertion into a vessel and treatment of both large and small lumens.